Roadway freezing point monitoring system and method

ABSTRACT

Improved roadway freezing point monitoring systems and methods include improved sample wells for the accurate measurement of the freezing point of liquid on a roadway, the use of temperature sensors that require only two conductors to receive power and to send and receive digital address and temperature information, improved algorithms for detecting the freeze point of liquid on the roadway, the use of conductivity measurements to verify detected freeze points, and the transmission of temperature information via the Internet to remote computers.

BACKGROUND

The application of freeze point depressants on roadways has long been amethod of combating the formation of ice. Traditionally, dedicatedmaintenance vehicles have applied anti-icing solid or liquid chemicalsto areas that have a high risk for developing ice. It is important toapply these anti-icing chemicals to the roadway before freezing occurs,as this prevents a bond from forming between ice and the roadway.Freeze-point depressants do this by depressing the freezing point of theliquid on the roadway, much as the anti-freeze in a car radiatorprevents it from freezing.

To do this well, a highway agency needs to know whether the current roadconditions warrant the application of chemicals. If the road surface hasan adequate concentration of chemicals for the current conditions, theapplication of additional freeze-point depressant is unnecessary,costly, and has an impact on the environment. Road Weather InformationSystems (RWIS) and their associated pavement sensors are onecost-effective way for highway agencies to monitor current roadconditions, without sending personnel into the field. Many RWIS systemscan send information on the current road conditions to a centralizedtraffic management center, where decisions on the application ofadditional freeze-point depressant can be made.

There are also some highway sites, such as bridges and overpasses, whichtypically freeze long before the rest of the roadway. Since the expenseof sending a truck with anti-icing chemicals to such a site is high,many highway agencies are installing fixed anti-icing systems. Thesesystems automatically determine the most opportune time to spray, basedon the current local conditions as reported by pavement and other RWISsensors. One of the most important parts of the RWIS system is thepavement sensor, as it allows the determination of the currentconditions of the roadway.

In its simplest form, the pavement sensor can consist of a thermometerthat measures the temperature of the road surface. Measuring thetemperature alone does not give enough information to determine if icewill form, however. This is because the exact concentration of theliquid present on the roadway is not known. For instance, the roadtemperature may be near 0° C., the freezing point of water. This maymean that the formation of ice is probable; however, previousapplications of anti-icing chemicals may have depressed the freezingpoint of the liquid on the roadway. Precipitation and its runoff mayalso have diluted the anti-icing chemicals previously applied to theroad. To most accurately gauge the current freeze point of the roadway,a sample of the actual liquid on the roadway needs to be analyzed. Onemethod of doing this is to freeze a small sample of solution on theroadway and determine its freezing point. Such a sensor is known as anactive sensor, because it actively changes the state of the liquid thatis on the road surface.

SUMMARY

The following sections describe a new active pavement sensor thatincludes unique features that increase the sensor's ability toaccurately predict the current state of the road.

By way of general introduction, the illustrated pavement sensors includeone or more of the following features, that can be used alone or incombination:

-   -   The illustrated freezing point sensor includes a sample well        that has a surface in good thermal contact with a thermal link        situated between the sample well and an active cooler. A        temperature sensor is disposed in good thermal contact with the        sample in this sample well.    -   The disclosed freezing point sensor confirms the freezing point        as measured with an active cooler and a temperature sensor by        additionally assessing the conductivity of the sample being        cooled.    -   The illustrated sensor module determines the freezing        temperature of a sample by measuring a freezing curve (a plot of        temperature. versus time, begun with the sample at a temperature        above its freezing temperature and continuing until the        temperature of the sample is below the freezing temperature),        and then assessing the shape of the freezing curve. One        disclosed algorithm locates a region of the freezing curve        having a slope that is level or slightly downwardly trending and        that occurs (1) after a second time derivative of the freezing        curve exceeds a threshold value or (2) after the first time        derivative of the curve exceeds a positive threshold value. The        disclosed system fits lines to multiple temperature measurements        in order to improve system performance.    -   The disclosed system uses two-conductor temperature sensors        having globally unique addresses. Power for the temperature        sensor and digital signals to and from the temperature sensor        are carried by a set of cables including no more than two        conductors.    -   Temperature information is transmitted from temperature sensors        having globally unique addresses to a base station, which        transmits temperature information via a network such as the        Internet to a remote computer.

This section has been provided only by way of general introduction, andit is not intended to narrow the scope of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a FIG. 1 is a perspective view of a pavement sensor modulethat incorporates a preferred embodiment of the invention.

FIG. 2 is a top view of a portion of the sensor module of FIG. 1.

FIG. 3 is a fragmentary cross-sectional view taken along line 3-3 ofFIG. 2.

FIG. 4 is a fragmentary cross-sectional view taken along line 4-4 ofFIG. 2.

FIGS. 5, 6, 7 and 8 are graphs illustrating operation of the sensormodule of FIG. 1.

FIG. 9 is a flow chart of a software routine included in the sensormodule of FIG. 1.

FIG. 10 is a more detailed flow chart of block 110 of FIG. 9.

FIG. 11 is a block diagram of a two-conductor temperature sensor.

FIG. 12 is a block diagram of a temperature monitoring system includingmultiple sensor modules of the type shown in FIG. 11.

FIGS. 13-19 are schematic diagrams of electrical circuits included inthe sensor module of FIG. 1.

FIG. 20 is a fragmentary cross-sectional view taken along line 20-20 ofFIG. 21.

FIG. 21 is a fragmentary top view of an alternative sensor module.

FIG. 22 is a fragmentary cross-sectional view taken along line 22-22 ofFIG. 23.

FIG. 23 is a fragmentary top view of another sensor module.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to the drawings, FIG. 1 shows an isometric view of an activepavement sensor module 10. The following sections will first describethe mechanical structure and electronics of the module 10, beforeturning to its measurement capabilities.

Description of Mechanical Structure of the Module 10

The exterior of the module 10 is formed by a lower housing 12 and acover 14. The lower housing 12 is connected via a cable 18 with a remotestation (not shown in FIG. 1), and the lower housing 12 is adapted to bemounted in a recess of a roadway such that the upper surface of themodule is substantially flush with the surface of the roadway. Simply byway of example, in one embodiment the lower housing 12 is about 5 inchesdiameter and 2 inches in height. In this example, the cover 14 isremovably mounted to the lower housing 12 to provide access to internalelectronics, and to facilitate service, calibration, and upgrades to theinternal electronics and software.

In this non-limiting example, the cover 14 is formed of a thermallyinsulative material having thermal properties and a color which aresimilar to that of the adjacent road surface (e.g. thermal conductivityof about 0.24 W/m·K). The use of such an insulative material for thecover 14 helps insure that the cover tracks the temperature of the road,as well as isolating a liquid collected in the cover during freezingpoint detection runs. In this non-limiting example, the lower housing 12is made of a thermally conductive material to facilitate the removal ofheat generated by the sensor module 10. Preferably, a ring of the sameinsulative material as that used for the cover 14 is secured to the topof the lower housing 12. This prevents the top cover 14 from becomingbonded to the grout material that is used to fix the module 10 in theroadway.

As shown in FIG. 2, the top cover 14 forms a sample cup 16 forcollecting a small amount of liquid from the road surface. The module 10includes a temperature sensor such as a thermistor 24 that is positionedadjacent to the sample cup 16 to measure the temperature of liquidcontained in the sample cup 16. In this example, two electricalconductivity probes 22 are mounted adjacent to the sample cup 16.

Also shown in FIG. 1 is an external temperature probe 300. Although thetemperature of the road can be measured via a thermometer that isembedded in the lower housing 12 of sensor module 10, a more accuratemeasurement can be made via a temperature sensor 302, inside of externalprobe 300. In this application, external probe 300 may be embeddedwithin an inch or so of the surface of the roadway being monitored, somedistance from the sensor itself. Alternatively, external probe 300 canbe embedded several inches or even several feet below the surface of theroadway to monitor the subsurface temperature of the roadway. Thisinformation is useful in algorithms that use the temperature profile ofthe roadway to estimate what the future surface temperature will be.

Using the digital two-conductor temperature sensors described below, thetemperature probe 300 can be reconfigured to include several temperaturesensors 302. These sensors would communicate and receive power over thesame two-conductor bus. In this configuration, the temperature probe 300can be lengthened so that it measures the temperature at a number ofdifferent locations in the roadway. Likewise, the temperature sensor 300can be located vertically, so that its internal sensors measure thetemperature profile of the road. In this way, the temperature probe 300can be configured so that the three dimensional temperature profile ofthe roadway is gathered, with all of the data and power for this networkbeing transferred over the same two-conductor bus.

FIG. 2 shows the relative size and location of the sample cup 16 as wellas the locations of a sample well 20 positioned over the thermistor 24and the two conductivity probes 22. The cross-sectional area A1 of thesample well 20 is substantially smaller than the cross-section area A2of the sample cup 16 in the view of FIG. 2. Though not required, in thisexample the sample cup 16 has a generally triangular shape in plan view.This shape provides the space needed for the conductivity probes 22 andthe thermistor 24 while minimizing the heat capacity of the sample cup16 and the contained sample that must be cooled during an active coolingoperation.

FIG. 3 is a cross section through the center of the sample well 20. Asshown in FIG. 3, the lower surface 36 of the sample cup 16 is formed byan aluminum plate that functions as a cold thermal link 32 with anactive cooler 30, e.g., a Peltier cooler. The cold thermal link 32 ispreferably formed of aluminum, because aluminum has a relatively highthermal conductivity and a relatively low heat capacity. The purpose ofthe cold thermal link 32 is to transfer heat into or out of the solutionin the sample cup 16, and a high thermal conductivity enhances theperformance of the cold thermal link 32. Similarly, a low heat capacityfor the cold thermal link 32 improves the response time of the link 32.The sample cup 16 also includes a surface 38 that is formed by theinsulating material of the cover 14. Thus, the sample cup 16 is formedas an opening 40 in the cover 14. A hot thermal link 34 is provided onthe opposite side of the active cooler 30 from the cold thermal link 32,and the hot thermal link 34 is used to transfer heat from the activecooler 30 to the environment.

As also shown in FIG. 3, the sample well 20 is positioned immediatelyover a housing 26 that mounts the thermistor 24. A first surface 42 ofthe sample well 20 is in good thermal contact with the housing 26 and,therefore, with the thermistor 24. A second surface 44 of the samplewell (which preferably extends around at least one half of thecircumference of the well 20, more preferably around 75% of thecircumference of the well 20, and most preferably around the entirecircumference of the well 20) is in good thermal contact with the coldthermal link 32. In particular, the thermal conductivity of the coldthermal link 32 closely adjacent the second surface 44 of the samplewell 20 is preferably greater than 1 W/m·K, more preferably greater than5 W/m·K, more preferably greater than 50 W/m·K, and most preferablygreater than 100 W/m·K. The cold thermal link 32 may have a thin layerwith a lower thermal conductivity (e.g., a corrosion layer) immediatelyadjacent the second surface 44 without materially adversely affectingheat flow from the sample in the well 20 to the cold thermal link 32,and the conductivity values given above are for the bulk material of thecold thermal link 32. The thermistor 24 is thermally isolated from thecold thermal link 32 by an insulating washer 28. This arrangementisolates a small portion of the liquid that is being frozen in a freezepoint detection run. The liquid in the sample well 20 is in good thermalconduct with the thermistor 24, and it is also in good thermal contactvia the second surface 44 with the cold thermal link 32. In thisexample, the second surface 44 completely surrounds the sample well 20on all sides. The thermistor 24 is less strongly linked to the coldthermal link 32 because it is partially insulated by the washer 28.

In this example, the thermistor housing 26 is formed of aluminum havinga thermal conductivity of about 200 W/m·K. The use of aluminum reducesthe thermal mass of the housing 26 and decreases the response time ofthe thermistor.

Though not shown in FIG. 3, the leads for the thermistor 24 arepreferably tightly wrapped around the outside of the thermistor housing26, typically about three times. This provides a heat sink for theleads. Since the leads are typically made from a highly thermallyconductive material such as copper, heat sinking them to the conductivehousing 26 mitigates any heat flow through them. This reduces the flowof heat to the thermistor 24 via the leads and reduces erroneously hightemperature measurements.

Also not shown in FIG. 3 is a potting material that is added to the areabelow the thermistor 24. In this example, the complete area beneath thethermistor 24 is filled with a high strength, low thermal conductivityepoxy. This fixes the location of all of the components, adds strengthto the cover 14, and reduces leaks.

FIG. 4 provides a cross section through the center of the conductivityprobes 22. These probes 22 are electrically and thermally isolated fromthe cold thermal link 32 that forms the bottom of the sample cup 16 by athermally and electrically insulating washer 45 at each end of eachconductivity probe 22. The conductivity probes in this example are heldin place by attachment nuts 47. The entire assembly is then encased bythe epoxy potting material described above.

Many materials can be adapted for use in the module 10. By way ofexample, the materials of Table 1 have been found suitable. TABLE 1Element Suitable Material and Dimension Cover 14 MDS filled Nylon 6/6Probes 22 Stainless Steel Housing 26 6061-T6 Aluminum Washer 28 Nylon6/6, 0.063 inch thick Cold Link 32 6061-T6 Aluminum, 0.063 inch thickHot Link 34 6061-T6 Aluminum Epoxy Scotch-Weld 1838-L B/A Epoxy

The sample well 20 captures a small amount of the liquid that is in thesample cup 16. This well 20 enhances heat flow into the water directlyabove the thermistor 24, by lessening the distance between the coldthermal link 32 and this sample of water.

The use of a thermistor well 20 has other benefits, in addition tobetter thermal conduction. The cold thermal link 32 cools much morerapidly than the water directly over the thermistor 24. This promotesfreezing of the water over the link 32, which then provides seedcrystals, allowing the water over the thermistor 24 to freeze with lesssupercooling. This pre-freezing of the water over the link 32 providesanother advantage in that it protects the sample in the thermistor well20 from splashes created by passing vehicles.

Description of the Electronics of the Module 10

In this non limiting example, the sensor module 10 includes thefollowing major components.

Controller

A programmable controller provides the control and analysis capabilityto the system. It includes an 8-bit microprocessor running at 18.432MHz, 256K of flash memory, and 128K of RAM. The controller may beimplemented as a Z-World Rabbit Core, Model RCM2020, programmed viaDynamic C (FIG. 13). The controller is responsible for monitoring theroad temperature, road moisture, and sample cup temperature. It alsocalculates the freeze point of liquid on the roadway and whether it isappropriate to issue a dew or frost warning. It communicates to themaster controller via the daughter board's RS-485 transceiver.

The controller can execute the program of attached Appendix 1. Appendix1 is made up of ASCII records of the following format::NNAAAATTDD₁DD₂DD₃ . . . DD_(N)CC

The colon starts every record. Each letter represents a hexadecimalnibble with the following meanings.

NN—Number of data bytes in record. For Dynamic C generated hex files,this is always either 02 for extended address records, 20 for datarecords, or 00 for EOF records.

AAAA—16 bit address. This is the offset portion off the destinationaddress using the Intel real-mode addressing. The segment portion of thereal-mode address is determined from the extended address record in thefile previous to the data record. The physical offset into the memorydevice is computed by shifting the segment left 4 bits and adding theoffset.

TT—Type of record. For Dynamic C generated hex files, this is alwayseither 02 for extended address records, 00 for data records, or 01 forEOF records.

DD_(i)—Data byte

CC—8 bit checksum of all previous bytes in the record. The two'scomplement of the checksum is used.

Appendix 1 includes copyrighted material, and the copyright holderhereby reserves all rights in Appendix 1, other than the right toreproduce Appendix 1 as part of this specification.

Daughter Board Electronics

The daughter board electronics include an A/D converter, current driversfor the sensor thermistor and conductivity probes (FIGS. 14, 16, 18 and19), an RS-485 transceiver circuit, a dip switch array to allow eachsensor to have an address, a drive circuit for the Peltier cooler (FIG.15), and power regulators to regulate the incoming voltage (FIG. 17).Also included on the daughter board is an interface circuit (FIG. 13) toexternal two-conductor digital temperature sensors. These temperaturesensors can be used to measure the road surface temperature and ifnecessary other parameters, such as the subsurface temperature.

Sensor Cup Temperature Measurement

The daughter board measures the sample cup thermistor 24 via a precisioncurrent driver and a 16-bit A/D converter (FIGS. 14, 18 and 19).

External Two Conductor Temperature Measurement

In this non-limiting example, external temperatures are measured usingDallas Semiconductor Corporation's digital two conductor temperaturesensors (known as “1-wire sensors”). These sensors are an example oftwo-conductor sensors because they only require two conductors totransmit both data and power. These sensors derive their power from thedata line, whenever it is held in its high state. Additionally, each ofthese sensors has its own globally unique address. This means that manysensors can be placed on the same two-conductor bus. Also, since thesensors are digital, they can be located remotely (up to and even morethan three hundred meters) from the host sensor.

These three features (a simple two-conductor bus, globally uniqueaddresses, and digital communication) give these sensors an economicadvantage over more traditional sensing techniques, such as individuallyconnected sensors. For instance, a series of five of these sensors canbe used to measure the temperature of the road at five different depths.Another application is the measurement of the road surface temperatureat a number of locations. More traditional sensors each require theirown wires and their distance from the host sensor is limited if theyproduce analog signals. Digital sensors exist that communicate over abus, but a method is required (i.e., an address) to differentiate thesensors. These sensors also typically require separate power circuits.The amount of wire and the number of conductors required are asignificant part of the design as the cost of wire for long runs canexceed that of the sensors that are at the end of the wire.

Electrical Conductivity Measurement

The resistance between the two conductivity probes 22 is measured via aprecision current driver and the second channel in the 16-bit A/Dconverter (FIG. 16). This circuit measures the conductivity of anyliquid that is present in the sensor cup 16. The drive current betweenthe probes 22 is reversible via an analog switch, limiting thepolarization of the conductivity probes 22 and the liquids on their topsurface. This circuit has shown that it can differentiate between a drysurface, a surface lightly coated with distilled water, and a surfacecovered with a salt solution.

Peltier Power Circuit

The Peltier power circuit turns on the Peltier cooler 30, as directed bythe controller (FIG. 15). Included in this circuit is an H-bridge toallow the Peltier cooler 30 to be heated or cooled, as appropriate.

Sensor Measurement Capabilities

The following four parameters are measured by the sensor module 10:

road surface temperature,

road surface moisture conductivity,

temperature of a sample of liquid in the sample well 20,

subsurface temperature (optional).

These parameters are used by the controller to determine the freezepoint of liquid in the sample well, as well as to provide dew and frostwarnings.

Freeze Point Algorithm

During the determination of the freeze point, the onboard controllerstores the temperature of the liquid in the sample cup versus time. Thisdata is then analyzed to determine the freezing temperature of theliquid, as described below. The algorithms described below assess theshape of the resulting freezing curve, and surface conductivity is usedas a verification of the freezing of the sample.

Dew Warning Algorithm

If the ambient temperature is near freezing and the road is dry, thePeltier cooler can cool the sample by several degrees. If moisture isthen detected, the sensor will give a dew warning, indicating theimpending formation of dew, and possibly black ice.

Frost Warning Algorithm

If the ambient temperature is below freezing and the road is dry, thePeltier cooler can cool the sample cup by several degrees. It can thenbe heated back to the ambient temperature. If moisture is then detected,the sensor will give a frost warning, indicating the impending formationof frost.

Description of Methods to Determine the Freeze Point

The following section describes the method used by the sensor module 10to detect the freeze point of a liquid. This method searches for aconstant temperature condition that exists during freezing. It does thisby continuously fitting a series of lines to the temperature versus timedata. It also continuously monitors the electrical conductivity of thewater to determine if freezing has occurred.

Discussion of the Data Gathered

As described above, this method makes use of the fact that the coolingcurve of a freezing liquid is nearly level. FIG. 5 shows a cooling curvefor a liquid whose freezing temperature is near −0.5° C. This data wasobtained during an actual freeze point detection run with the sensormodule 10. The cooling curve in FIG. 5 is nearly level between zero andfive seconds as the Peltier cooler 30 works to cool the thermal mass ofitself, the cold thermal link 32 and the sample of liquid in the well20. There is also a delay in the response because the thermistor 24 isnot strongly linked to the Peltier cooler 30. At about five seconds, thecurve begins to trend downwards, until at about ten seconds, where ithas reached a nearly constant slope. The curve continues at thisconstant slope until about 17.5 seconds. At this point, the liquid hasbeen supercooled to −1.2° C., which is below its nominal freezingtemperature of −0.5° C. The cooling curve then slopes upward until about21 seconds, where it levels off around the freeze point of −0.5° C.

It should be noted that there are variations to the shape of the coolingcurve, as described above and shown in FIG. 5. As an example of thisvariation, FIG. 6 shows four cooling curves, measured with the sensormodule 10. The cooling curve shown in FIG. 5 is labeled “Cooling Curve1” in FIG. 6. In three of the cases shown in FIG. 6 (cooling curves 1,3, and 4), the cooling curves show the liquid being substantiallysupercooled, prior to solidification. Cooling curve 2, however, showsvery little supercooling, prior to the leveling out which is indicativeof freezing. Solidification without substantial supercooling has beenobserved under a variety of conditions with liquids having a range offreezing temperatures.

FIG. 6 also shows the different slopes the cooling curve can take, oncesolidification has begun. For cooling curves 1 and 2, which are nearlypure water, the curves are nearly level, once solidification has begun.For cooling curves 3 and 4, which were gathered from salt solutions thatdiffer in both type and concentration, the cooling curves slopedownwardly during solidification. This is because the water in asolution freezes first, increasing the concentration and lowering thefreezing point of the remaining solution.

It should be noted that the shapes of the curves, including the slopes,the amount of supercooling, and the temperatures obtained duringfreezing, are highly dependant upon the design of the sensor itself. Forexample, changing the heat capacity, conductivity, or geometry of any ofthe components, or changing the Peltier cooler 30 or the characteristicsof the Peltier cooler power source, will change the shape of the curvesobserved. There are numerous other changes that can be made to thedesign of the sensor that will change the shape of the cooling curves.

FIG. 7 compares the cooling curve of FIG. 5 with the measurements madeby the pavement sensor's conductivity probes 22. The actual values ofthe conductivity are dependant upon many factors, including thetemperature, the solution type, the geometry and construction of theconductivity probes 22 and for how long they are sampled. What issignificant about the conductivity data in FIG. 7 is that theconductivity of the solution falls significantly during solidification.

Presently Preferred Algorithm

The freeze-point detection algorithm analyzes the temperature versustime data acquired from the thermistor 24 by fitting a series of linesto the preceding six data points. FIG. 8 shows the output from thiscurve fitting routine. The sample cup thermistor data that is shown isthe same as was shown in FIGS. 5, 6, and 7. Six of the data points inthis line are highlighted to represent the points that would be used forone of the line fits. The last of these points, at 25 seconds,represents the most current data point in this line fit. Once the linehas been fit, a second line is formed from the slopes of the precedingsix line fits. This results in a trend of the previously calculated lineslopes, or a “slope of the slope.”

The three upper lines in FIG. 8 represent the slope of the line, b, thescatter of the line fit, s*10, and the “slope of the slope”, b′. Theparameters are compared to preset values to make a determination offreezing. Two separate algorithms are run simultaneously to make thisdetermination. The first of these looks for a positive slope in thecooling curve to indicate warming due the release of latent heat. Thesecond of the algorithms does not require a positive slope, but insteadlooks for a significant change in the slope as given by b′, the “slopeof the slope.”

The conductivity measurement is used as a verification that freezing hasoccurred. Use of the conductivity alone as an indicator of freezingwould result in an unreliable measurement. This is because theconductivity probes are not necessarily the same temperature as thewater in the thermistor well. The measured conductance can be used asverification.

The slope b can be taken as an example of a first time derivative of thetemperature measurements, and the slope of the slope b′ can be taken asan example of a second time derivative of the temperature measurements.

Description of the Algorithm

The following paragraphs describe the actions that are taken by thesensor algorithm to determine the freeze point of a liquid in thethermistor well.

1. Take a current sample cup temperature reading.

2. Fit a line to the last 6 sample cup temperature data points usinglinear regression. The line has the equationT=a+b·t,where a and b are constants, T represents temperature, and t representstime. The fitting equations for the constants a and b are as follows:$b = \frac{\sum\limits^{\quad}\quad{\left( {t - \overset{\_}{t}} \right) \cdot \left( {T - \overset{\_}{T}} \right)}}{\sum\limits^{\quad}\quad\left( {t - \overset{\_}{t}} \right)^{2}}$anda={overscore (T)}−b·{overscore (t)},where {overscore (t)} and {overscore (T)} are the average values of tand T for the six data points being used.

The goodness of fit, s2, is computed by the equation${s^{2} = \frac{{\sum\limits^{\quad}\quad T^{2}} - {a \cdot {\sum\limits^{\quad}\quad T}} - {b \cdot {\sum\limits_{\quad}^{\quad}\quad{t \cdot T}}}}{\left( {m - 2} \right)}},$where m is the number of data points used in the fit. This line fitstatistic is reduced to the “s*10” statistic for convenience in thealgorithm by the relation,s·10=10{square root}{square root over (s ² )}.

3. Compute the “slope of the slope” by fitting a line to the last 6 lineslopes obtained, using the following formula:${b^{'} = \frac{\sum\limits^{\quad}\quad{\left( {t - \overset{\_}{t}} \right) \cdot \left( {b - \overset{\_}{b}} \right)}}{\sum\limits^{\quad}\quad\left( {t - \overset{\_}{t}} \right)^{2}}},$where b′ is the “slope of the slope” and the values of b are the slopesfrom the last 6 fitted lines.

4. Determine from the fitting constants whether the freezing temperaturehas been reached. For the sensor module 10, the following rules wereapplied to the fitted line constants to determine when the freezingtemperature had been reached. These rules were based on thecharacteristics of this particular device. Other devices would mostlikely have different values for these criteria. Two methods arecurrently used simultaneously, one that looks for a transition in thecurve due to a supercooled fluid and one that assumes no appreciablesupercooling.

4.1. For the supercooling routine, no data is considered for the firstseconds, so that only reliable fits are considered. After this, thecurrent slope is constantly monitored. When the slope rises above 0.5, alogical variable “Freeze_Start” is set to TRUE, indicating that freezinghas begun. This indicates that subsequent line fits should be consideredas possible plateaus in the freeze curve. It also sets the Peltiercooler 30 at a reduced power setting, where the Peltier cooler 30 isswitched on and off. In the current design, the Peltier cooler 30 is setat a duty cycle of 50 percent once freezing has begun.

4.2. For the non-supercooling routine, no data is considered for thefirst 10 seconds. This allows for reliable fits and also eliminates theinitial level portion of the freeze curve. After this, the current slopeis constantly monitored. When the slope is greater than −0.1, a logicalvariable “Slope_Start” is set to TRUE. This indicates that freezing mayhave begun, however the Peltier cooler 30 continues at full power. Nextthe b′ parameter, or slope of the slope, is checked. When this parameteris greater than 0.05, freezing is determined to have begun and the“Freeze_Start” variable is set to TRUE. Subsequent line fits are thenconsidered as possible plateaus in the freeze curve and the Peltiercooler 30 is set a reduced power setting, as is described at 4.1.

4.3. Once one of the above conditions has been met (and “Freeze_Start”has been set to TRUE), the parameters of the current line fit arechecked to see if they fall within preset bounds that are indicative offreezing. Currently a value of s*10 that is less than 0.5 and a value ofb greater than −0.12 and less than or equal to 0.0 are used to indicatethat the plateau has been reached. Testing for values of bin this rangeare to be understood as one way of testing whether the cooling curveT(t) has leveled off. Thus the term “leveled off” is intended to includeslopes of T(t) that are somewhat negative, such as the slopescharacteristic of freezing of a salt solution. However, large negativeslopes, such as those associated with active cooling after a splash ofwater has entered the sample well, are excluded.

4.4. If the parameters of step 4.3 are met, the sensor module 10 usesthe first data point in the line fit as the determined freeze point.This ensures that the highest temperature is used for concentratedsolutions that have a steep freezing curve.

Referring again to FIG. 8, the decisions that the algorithm makes areshown in the timeline of Table 2. For simplification; an “x” in Table 1has replaced data that is not relevant to a particular algorithm step.TABLE 2 Time Temp. (sec.) (C.) b S*10 b′ Algorithm Action  5.0 1.4 −0.04x x Initial data has been gathered. Line slopes begin to be reviewed forfreezing by the “Supercooled” algorithm. The current line slope does notindicate that the latent heat from supercooling is being released.Freeze_Start remains FALSE. 10.0 0.7 −0.18 x −0.03 Initial data has beengathered and the sensor has been given enough time for a down slope tobegin. The “Non- supercooled” algorithm begins to check for leveling offwhich would be due to freezing. The current b′ does not indicatefreezing, so Slope_Start remains FALSE. The “Supercooled” algorithmcontinues to check for a freeze point. The current line slope does notindicate that the latent heat from supercooling is being released.Freeze_Start remains FALSE. 18.5 −0.9   −0.06 1.46 0.07 The slope isgreater than −0.1, so the “Non-supercooled” algorithm sets Slope_Startto TRUE. The value of b′ is also greater than 0.05, so the Non-Supercooled algorithm also sets Freeze_Start to TRUE, causing thePeltier to lessen its cooling. Because Freeze_Start is TRUE thedetection algorithm checks for a plateau, but ignores this data point,because the s*10 value indicates that the data has a high amount ofscatter. 17.0-20.0 Xx x x x The conductivity of the sample above thecooling plate goes high (see FIG. 7), indicating that freezing in thethermistor well is imminent 25.0 −0.5   0.00 0.10 x The line slope (b)is reduced to zero and the s*10 scatter is less than 0.5, so detectionalgorithm logs the first data point in the line fit, −0.47° C., as thedetermined freeze point.

FIG. 9 and 10 provide flow charts of software routines that implementthe freezing point detection algorithms discussed above. The routine ofFIG. 9 first checks the logical variable FindFreeze. If the ambient roadtemperature is within an operational range, e.g., between +5 and −15°C., and if the surface conductivity indicates the presence of water, theparameter FindFreeze is set to TRUE. Only in this case is controltransferred to block 102, in which the active cooler 30 is turned on tobegin cooling liquid in the sample cup 16 and the sample well 20. Thetemperature indicated by the thermistor 24 is read repeatedly, and onceit has been determined that freezing has begun, the active cooler 30 isoperated at a slower cooling rate, e.g., at 50 percent duty cycle, inblock 106. In block 108 the last six temperature measurements are fit toa line and the parameters b, s*10, and b′ discussed above arecalculated. These calculated parameters are then analyzed in block 110to determine whether a freeze point was found, and the result isreported in blocks 112 and 114. If a freeze point is found, the cooler30 is turned off. If the freeze point is substantially lower than theambient temperature, the cooler 30 will continue to cool until it coolsbelow the minimum cooling temperature (e.g., −15° C.). At this point,the controller will turn off the cooler 30 and discontinue thefreeze-point detection run. The minimum cooling temperature is thenreturned as the freeze point. The controller will also stop the run ifthe measured temperature provided by the thermistor 24 falls more than10 degrees below the ambient temperature. If there is substantialsplashing by passing vehicles, the sensor module 10 may need extra timeto complete the freeze-point detection run. If a freeze-point detectionrun is not completed within the time specified by a stored constant(e.g., 5 minutes), the controller will turn off the active cooler 30 anddiscontinue the run. The controller will continue to return the lastdetected freeze point until the master controller has indicated that ithas received this value by returning a normal data request instead of afind freeze point request.

FIG. 10 provides further information as to operation of block 110 ofFIG. 9. In block 150, variable “Freeze_Start” is set to TRUE if thenumber of samples is greater than 20 and if the line slope b is greaterthan 0.5. These conditions are typically met when a supercooled liquidbegins to freeze. In block 152, the logical parameter “Freeze_Start” isset to TRUE if the number of samples is greater than 20, the line slopeb falls to a value below −0.1 and then the slope of the slope b′ risesto a value above 0.05. These conditions are typically met by theinitiation of freezing in a non-supercooled liquid. In block 154, theparameter “Freeze_Start” is checked, and if it is in the TRUE state, theparameter s*10 is checked and the line slope b is checked to determinewhether it is less than zero and greater than a negative threshold(−0.12 in this non-limiting example). If so, the logical variableFreezeFound is equal to TRUE. The parameters checked in block 154 arecharacteristically met when a freezing sample reaches a temperatureplateau that is either level or tending downward slightly.

Though not required, the method of FIGS. 9 and 10 can be supplemented bychecking the conductivity measurements around the time of freezing asindicated above. For example, if a conductivity measurement shows asharp decrease in conductivity at about the time the freezingtemperature is reached as indicated by the thermistor, this can be takenas a confirmation that in fact freezing has occurred.

As mentioned above, when the freezing point detection algorithmindicates that freezing has commenced, the earliest temperaturemeasurement (which is typically the highest temperature) within thesamples used to determine the slope b is selected as the freezing pointtemperature.

Two Conductor Devices

The two-conductor devices described above can be constructed as shownschematically in FIG. 11, and suitable devices can be acquired fromDallas Semiconductor Corporation, a subsidiary of Maxim IntegratedProducts, as Model No. DS18B20._. In FIG. 11, a two-conductor device 200includes a power circuit 202 that draws power from a conductor 204 andsupplies this power to the remaining components of the device 200. Thedevice 200 also includes a random access memory 208, a memory controllerlogic 210, and a temperature sensor 212. The read only memory 206 storesa globally unique address, e.g., a 64-bit address. The temperaturesensor 212 operates to store a temperature measurement in the randomaccess memory 208. The memory controller 210 transmits addressinformation from the read-only memory 206 via the conductor 204 as wellas temperature information from the random access memory 208 via theconductor 204. Both the address information and the temperatureinformation are transmitted as serial, digital signals. Typically, aground conductor 214 is also connected to the device 200, and the twoconductors 204, 214 serve to transmit power to the device 200, digitalsignals to the device 200, and digital address and temperature signalsfrom the device 200. Also shown in FIG. 11 is an alternate powerconductor 215 that can also be used to power the device 200. Ifconductor 204 is used to power device 200, power conductor 215 is tieddirectly to ground conductor 214.

FIG. 12 shows one example in which many two-conductor devices 200 areconnected via a two-conductor cable 219 to an I/O module 223 of a basestation 221 that also includes a network server 220. The network server220 is connected via an Internet connection 222 to a remote web-basedbrowser 224, that will typically be implemented on a remote computer.The Internet connection 222 can take any suitable form, such as awireless connection, a direct line connection, or a dial-up connectionto an Internet service provider. The network server 220 and the devices200 exchange both temperature information and address information asserial, digital signals on the conductors of the cable 219. As explainedabove, only two conductors are required to bring both power and digitalsignals to and from each of the devices 200.

In this example, the read-only memory 206, the random access memory 208,and the memory controller logic 210 operate as a means for transmittingtemperature and address information from the device 200 to the networkserver 220. These components can be implemented in any desired fashion,and the present invention is not limited to any particular type ofcontroller logic or memory. Similarly, the network server 220 operatesas a means for transmitting temperature information from the basestation that houses the network server 220 to the remote computer thathouses the web-based browser 224. With this arrangement, the user canaccess via the Internet temperature information measured by any of thedevices 200 of FIG. 12.

Many alternatives are possible. For example, other networks can be usedin substitution for the Internet network described above. The Internetprovides important advantages, in that it reduces the cost andinconvenience of remotely accessing information provided by the devices200. The devices 200 are not limited to temperature measuring devices,and they can include other types of sensors, e.g., conductivity sensorsand other sensors based on A/D converters, as well as counters ofvarious types.

Conclusion

Of course, it should be understood that many changes and modificationscan be made to the preferred embodiments described above. For example,many changes can be made to the shape of the sample well and adjacentelements. FIGS. 20 and 21 show cross-sectional and top views,respectively, of part of a sensor module 400 that includes a cover 402having an opening 404 that defines a sample cup. A cold thermal link 406forms the bottom of the sample cup, and the cold thermal link partiallysurrounds a sample well 408. A first surface 412 of the sample well 408is in good thermal contact with a temperature sensor 410, and a secondsurface 414 of the sample well 408 is in good thermal contact with thecold thermal link 406. FIGS. 22 and 23 show corresponding views of asensor module 500 having a cover 502 having an opening 504 that definesa sample cup. A cold thermal link 506 forms the bottom of the samplecup, and the cold thermal link 506 partially surrounds a sample well508. A first surface 512 of the sample well 508 is in good thermalcontact with a temperature sensor 510, and a second surface 514 of thesample well 508 is in good thermal contact with the cold thermal link.As should be apparent from these figures and FIGS. 1-3, the sample wellcan take many shapes, and the first and second surfaces can be orientedat various angles. The first and second surfaces can be planar,cylindrically shaped, or otherwise curved. For example, the first andsecond surfaces may be separate parts of a single hemispherically shapedrecess that defines the sample well.

Also, the two-conductor device 200 and the Internet accessible system ofFIG. 12 can be used with sensor modules having other types ofmeasurement zones that do not, for example, include a sample well 20 asdescribed above, that use other algorithms for freeze-point detection(e.g., prior art freeze-point detection algorithms or that measuretemperature passively). Furthermore, this invention is not limited tothe use of thermistors for temperature sensors, and if desired, othertemperature sensors such as thermocouples and other temperaturesensitive elements can be substituted.

As used herein, the term “time” is intended broadly to encompassabsolute or relative measures of time. The term “time derivative” isintended broadly to encompass time differences, slopes, slope of slopesand other measures of the rate of change of a variable such astemperature, whether averaged or not, whether discrete or continuous,and whether numerically or analytically determined.

The term “conductivity” is intended broadly to encompass any measurethat varies as a function of the resistance between two probes, whetherthe measured parameter is current, voltage or some combination thereof,and whether it varies directly or inversely with resistance, and whethermeasured with DC or AC voltages.

The term “temperature information” is intended broadly to encompassfreezing point temperature as determined with an active cooler, ambienttemperature, or other temperature parameters. Further, the term“freezing point temperature” refers to a chosen point in the temperatureversus time curve, once solidification has begun, or is about to begin.It is not limited to points at the beginning of solidification of thesample, but can be any appropriate point in the curve.

The term “good thermal contact” is intended broadly to signify that thethermal conductivity between two elements is at least 1 W/m·K.

The foregoing detailed description has discussed only a few of the manyforms that this invention can take. This detailed description istherefore intended by way of illustration, and not by way of limitation.It is only the following claims, including all equivalents, that areintended to define the scope of this invention.

1. A roadway freezing point sensor comprising: a sensor moduleconfigured to be embedded in a roadway, said sensor module comprising anactive cooler, a cold thermal link in thermal contact with the activecooler, a sample well adjacent the thermal link, and a temperaturesensor adjacent the sample well; said sample well comprising a firstsurface in good thermal contact with the temperature sensor and a secondsurface in good thermal contact with the thermal link; said thermal linkhaving a thermal conductivity greater than 1 W/m·K closely adjacent thesecond surface of the sample well.
 2. The invention of claim 1 whereinthe thermal link extends completely around the sample well.
 3. Theinvention of claim 1 further comprising: a thermally insulating coverdisposed over a portion of the cold thermal link spaced from the samplewell.
 4. The invention of claim 3 wherein the cover defines an openingpositioned above the sample well.
 5. The invention of claim 4 whereinthe sample well projects an area A1 in a horizontal plane, wherein theopening in the cover projects an area A2 in the horizontal plane, andwherein A2>A1.
 6. The invention of claim 5 wherein the opening forms asample cup having a lower surface in good thermal contact with the coldthermal link.
 7. The invention of claim 6 further comprising a pair ofconductivity probes in good electrical contact with a measurement zonebounded by the sample cup, said conductivity probes positioned alongsidethe temperature sensor.
 8. The invention of claim 7 wherein the samplecup comprises a surface formed by the cover.
 9. The invention of claim 1further comprising a thermally insulating element positioned between thetemperature sensor and the cold thermal link.
 10. The invention of claim1 wherein the thermal link has a thermal conductivity greater than 5W/m·K closely adjacent the second surface of the sample well.
 11. Theinvention of claim 1 wherein the thermal link has a thermal conductivitygreater than 20 W/m·K closely adjacent the second surface of the samplewell.
 12. The invention of claim 1 wherein the thermal link has athermal conductivity greater than 100 W/m·K closely adjacent the secondsurface of the sample well.
 13. A roadway temperature monitoring systemcomprising: a base station; a plurality of sensor modules embedded in aroadway, each sensor module comprising a digital temperature sensorhaving a respective address which is globally unique within thetemperature monitoring system; a set of cables interconnecting the basestation with the sensor modules, said cables comprising no more than twoconductors connected to each sensor module; said sensor modules eachcomprising a respective power circuit operative to draw operating powerfor the respective sensor module from the conductors; said sensormodules operative to transmit temperature information and addressinformation to the base station as serial digital signals via theconductors.
 14. A method for measuring a liquid freezing point on aroadway, said method comprising: (a) providing a sensor module embeddedin a roadway, said sensor module comprising a measurement zone on anupper surface thereof, a temperature sensor in good thermal contact withthe measurement zone, a pair of conductivity probes in good electricalcontact with the measurement zone, and an active cooler thermallycoupled to the measurement zone; (b) progressively cooling themeasurement zone with the active cooler; (c) recording temperaturemeasurements of the measurement zone with the temperature sensorrepeatedly during (b); (d) recording conductivity measurements of themeasurement zone with the conductivity probes repeatedly during (b); (e)determining a freezing point of a liquid in the measuring zone based onboth variations in time of the temperature measurements of (c) and theconductivity measurements of (d).
 15. The method of claim 14 wherein (e)comprises: (e1) determining the freezing point based on variations intime of the temperature measurements of (c); and (e2) checking whetherthe conductivity measurements of (d) indicate a substantial decrease inconductivity substantially coincident with the freezing point determinedin (e1).
 16. The method of claim 14 wherein the measurement zone of (a)comprises a sample well having a first surface in good thermal contactwith the temperature sensor and a second surface in good thermal contactwith the active cooler.
 17. The method of claim 15 wherein (e1)comprises: (e1a) determining a second time derivative of the temperaturemeasurements of (c); (e1b) comparing the second time derivative of (e1a)with a positive threshold value; and (e1c) determining the freezingpoint based on detection of a leveling off of the temperaturemeasurements of (c) after the second time derivative of (e1 a) hasexceeded the threshold value of (e1b).
 18. A method for measuring aliquid freezing point on a roadway, said method comprising: (a)providing a sensor module embedded in a roadway, said sensor modulecomprising a measurement zone on an upper surface thereof, a temperaturesensor in good thermal contact with the measurement zone, and an activecooler thermally coupled to the measurement zone; (b) progressivelycooling the measurement zone with the active cooler; (c) recordingtemperature measurements of the measurement zone with the temperaturesensor repeatedly during (b); (d) determining a second time derivativeof the temperature measurements of (c); (e) determining when the secondtime derivative of (d) exceeds a positive threshold value; and (f)determining the freezing point based at least in part on a leveling offof the temperature measurements of (c) after the second time derivativeof (d) has exceeded the threshold value of (e).
 19. The method of claim18 wherein (d) comprises: (d1) determining a plurality of first slopesof the temperature measurements of (c) versus time, each first slopeassociated with a respective measurement time; and (d2) determining aplurality of second slopes of the first slopes of (d1) versus time. 20.A method for measuring a liquid freezing point on a roadway, said methodcomprising: (a) providing a sensor module embedded in a roadway, saidsensor module comprising a measurement zone on an upper surface thereof,a temperature sensor in good thermal contact with the measurement zone,and an active cooler thermally coupled to the measurement zone; (b)progressively cooling the measurement zone with the active cooler; (c)recording temperature measurements of the measurement zone with thetemperature sensor repeatedly during (b); (d) determining a first timederivative of the temperature measurements of (c); (e) determining whenthe first time derivative rises above a first, positive threshold value;and then (f) determining when the first time derivative falls to withina range of values bounded above by zero and below by a second, negativethreshold value.
 21. The method of claim 20 wherein (d) comprises: (d1)fitting a plurality of lines to the temperature measurements of (c); and(d2) determining a first time derivative of the lines of (d1).
 22. Themethod of claim 21 further comprising: (g) associating the earliesttemperature measurement included in the line associated with a timedetermined in (f) as the liquid freezing point.
 23. A roadway monitoringsystem comprising: a base station; a plurality of sensor modulesembedded in a roadway and coupled with the base station, each sensormodule comprising a temperature sensor and a memory storing an addresswhich is globally unique within the monitoring system; means fortransmitting temperature information from the sensor modules to the basestation; and means for transmitting temperature information from thebase station via a network to a remote computer.
 24. The invention ofclaim 23 wherein the network comprises the Internet.